One example of a display device is a liquid crystal display device. In a liquid crystal display device, it takes liquid crystal a relatively long time to respond to an input of a video signal. If the response speed of a display device is slow, an image cannot be displayed in an input tone. To improve the response speed of liquid crystal, a liquid crystal display device adopts overdrive. Overdrive is, when a voltage which drives liquid crystal is to be changed, to make the voltage change drastic. For example, a higher voltage is applied when a voltage is to be increased, and a lower voltage is applied when a voltage is to be reduced. That is, overdrive aims to display an image having an original luminance at high speed by correcting the gray level of a video signal of the current frame to be displayed based on the difference between the gray level of the video signal of the current frame and the gray level of a video signal of the previous frame.